The units for producing carbon monoxide and hydrogen may be separated into two parts:                Generation of the syngas (a mixture essentially containing H2, CO, CH4, CO2, Ar and N2). Among the various industrial syngas production processes, the one based on coal gasification appears to be becoming more and more widespread especially in countries that are rich in coal deposits such as China. The process of partial oxidation of natural gas may also prove advantageous for the production of CO alone or with low H2/CO production ratios. Another process is steam reforming.        Purification of the smelts. Found therein are:        a scrubbing unit with a liquid solvent to remove most of the acid gases contained in the syngas;        a unit for purification over adsorbent beds;        a unit for cryogenic separation referred to as a cold box for the production of CO.        
In the case of a syngas resulting from a steam reforming furnace, for the production of CO and hydrogen under pressure, the most common cryogenic process is methane scrubbing, the residual methane content in the syngas being compatible with the methane scrubbing process. In certain cases, it is necessary to include a CO/N2 separation column in the cold box, especially when the nitrogen content in the natural gas is not compatible with the purity of the CO product without this CO/N2 column.
A methane scrubbing layout with CO/N2 column is described in FR-A-2910603.
In the case of certain coal gasification processes, the syngas produced is treated in a cold box by partial condensation with no cycle, the content of inert species (CH4, Ar and N2) being very low and compatible with the purity of the CO.
A partial condensation layout without a separation column is described in EP-A-1729077 and in FR-A-2930332.
A partial condensation layout with a CO/N2 column is described in U.S. Pat. No. 4,478,621. The reboiling of the CO/N2 column being provided by a direct feed of CO at medium pressure into the bottom of the column coming from the CO compressor.
DE-A-4228784 and DE-A-2147465 describe processes comprising the features of the preamble of claim 1 that are similar to that of FIG. 1.
According to FIG. 1, a stream of nitrogen and carbon monoxide 1 is cooled in a heat exchanger 3. It is at least partially condensed in a reboiler 5 fed by the bottoms liquid 17 from the column 15. The vaporized liquid 18 is sent back to the column. The at least partially condensed stream is expanded in a valve 7 and sent to a phase separator 9. The liquid 11 from the phase separator and the gas 13 from the phase separator 9 are sent to the column at different or identical heights. The bottoms liquid 21 from the column 15 is expanded in a valve 19 and sent to the overhead condenser 23 forming part of the column 15. A stream of nitrogen is removed as a purge 25 and is heated in the heat exchanger 3. The vaporized carbon monoxide 27 is also heated in the heat exchanger 3 and is compressed in the first stage 29 of a compressor. It is cooled in a cooler 31 by water and then divided into two. The stream 33 is cooled to an intermediate temperature in the heat exchanger 3 in order to form the stream 45 which is sent to the bottom of the column 15. The stream 41 is very partially cooled in the heat exchanger 3, expanded in a turbine 43 and remixed with the stream 27. The rest of the carbon monoxide 35 is compressed to product pressure by the stage 37 of the compressor and cooled by the cooler 31A in order to form the product stream 39. The stage 29 is even smaller since a part of the reboiling is provided by the stream 1.